I had a keg recently that i swore I'd never get the liquid post off, after looking at it a little closer the rubber was part way up the bolt. I tried everything and almost bought a socket set. I trimmed the rubber with a razor and after riding the keg and tapping my auto wrench with a hammer it finally came loose.
I've also saw a few threads where people stopped by a local auto shop and had them bust it loose with their air impact wrench. that was going to be my next step
If you do end up buying a socket, go to an auto parts store because depot wants $30 or $40 for a set with one. At least you can just buy the one you need